Many communication systems require the isolation of the carrier portion of the modulated wave. For example, in reduced carrier (A3A) transmission the carrier is transmitted along with the sideband frequencies at a reduced amplitude, generally 10 to 20 db below the peak sideband level. In many situations, this carrier wave is used to demodulate the sideband signal in a product demodulator. This insures that the recovered frequency components will be accurate and will protect the system against frequency errors due to the frequency doppler shifts. Such systems have been in use for communication for many years and have generally given good service.
One of the problems connected with this type of transmission system is that it is necessary that the band width of the isolating filter be wide enough to follow drift and frequency errors but narrow enough so that desired sideband components do not disturb the performance of the system. This requires, in single sideband systems, that the carrier filter pass only the desired frequency plus or minus a shift of 100Hz. In most transmitters which carry voice frequencies, the frequency response is restricted so that the lowest audio frequency is greater than 300 Hz. If the transmitter filter circuits are sufficiently selective and are stable, this system works fairly well.
However, there are some cases where the low pass filter has changed, due to temperature changes or aging, and there is some response at frequencies as low as 50 Hz. Also, there are cases where the carrier frequency drifts to the edge of the sideband filter, thereby permitting the low frequency components of the sideband to fall within the passband of the carrier filter. In this case, the sideband components will produce objectional phase modulation components and introduce distortion. The invention described herein greatly reduces this problem, making such systems relatively free of such difficulties.
The invention is based upon the fact that frequency division or frequency multiplication does not change the spacing of the sideband frequencies relative to the carrier. For example, if a single-sideband transmitter is modulated with a 100 cycle tone, the lowest separation between the sideband and the carrier is 100 Hz. This separation is maintained no matter how many times the signal is multiplied or divided in frequency.
In the Marine radio telephone field, a specification of .+-. 100 cycles per second for the accuracy of the carrier frequency has been standardized for certain types of modulation. For this reason, the band width of the carrier channel must be at least .+-. 100 Hz in order to handle the transmissions properly. Since it is possible that the transmitter carrier wave be at the edge of its tolerance, say 100 Hz low, upper sideband components at 200 Hz from the carrier just pass the selectivity requirement. These sideband components may be fairly strong and, since in the A3A transmission the carrier is at least 10 db below the sideband, it is possible that distortion may result.
The basic idea of the invention is to divide the frequency of the carrier frequency wave. Frequency division reduces the amount of frequency error which is permissible in the system. The carrier wave is first filtered by a crystal filter having a band width of .+-. 100 Hz. Then, the resulting wave is passed through a limiter to remove all traces of amplitude modulation and then the carrier frequency is divided by 8 by any of the known division circuits. If the original carrier had a frequency of 100 KHz, the divided wave has a frequency of 12.5 KHz. A frequency error at the input frequency of 100 Hz will now be an error of 12.5 Hz. The band pass filter which lies between the dividing and multiplying circuits should have a band width of only .+-. 12.5 Hz. An ordinary band pass circuit filter can be used for this purpose, but it is believed that a phase locked loop is preferable. Other active circuits, such as a locked oscillator, have been considered, but it is believed that the phase locked loop is the best. The phase locked loop as used in this situation acts like a tracking filter which follows frequency errors within the range of .+-. 12.5 Hz; however, due to the circuitry that restricts its speed, effectively cuts off undesired sideband components.
Additional details of the invention will be disclosed in the following description, taken in connection with the accompanying drawings.